The present invention relates to the diagnostic imaging arts. It particularly relates to computed tomography imaging employing an x-ray source and a two-dimensional detector array that enables rapid acquisition of volumetric x-ray absorption imaging data, and will be described with particular reference thereto. However, the invention will also find application in other types of radiation detectors for a variety of imaging applications employing x-rays, visible light, radiation from an administered radiopharmaceutical, or other types of radiation. The invention will further find application in non-imaging radiation detectors.
Computed tomography (CT) imaging typically employs an x-ray source that generates a fan-beam, wedge-beam, or cone-beam of x-rays that traverse an examination region. A subject arranged in the examination region interacts with and absorbs a portion of the traversing x-rays. A one- or two-dimensional radiation detector including an array of detector elements is arranged opposite the x-ray source to detect and measure intensities of the transmitted x-rays.
Typically, the x-ray source and the radiation detector are mounted at opposite sides of a rotating gantry such that the gantry is rotated to obtain an angular range of projection views of the subject. In some configurations the x-ray source is mounted on the rotating gantry while the radiation detector is mounted on a stationary gantry. In either configuration, the projection views are reconstructed using filtered backprojection or another reconstruction method to produce a three-dimensional image representation of the subject or of a selected portion thereof. Typically, the reconstruction assumes that the radiation traversed a linear path from the x-ray source directly to the detector. Any scattered radiation that reaches the detector degrades the resultant image.
The detector array of the radiation detector typically includes a scintillator crystal array which produces bursts of light, called scintillation events, in response to x-rays. A two-dimensional array of photodetectors such as a monolithic silicon photodiode array are arranged to view the scintillator and produce analog electrical signals indicative of the spatial location and intensity of the scintillation event. The intensity is typically translatable into an energy of the x-ray photon that produced the scintillation event, and hence provides spectral information.
Typically, the detector array is a focus-centered array including a curved detection surface defining a focus that coincides with a focus of the x-ray beam which is typically at or near the x-ray source. Preferably, anti-scatter elements such as arrays of anti-scatter plates are mounted in front of the scintillator, and are precisely aligned with the x-ray paths to block scattered x-rays which would otherwise contribute to measurement noise. The spacing between the anti-scattering plates defines slits through which the direct or non-scattered x-rays pass unimpeded. However, scattered x-rays are angularly deviated due to the scattering and strike the anti-scatter plates which absorb the scattered x-rays.
The anti-scatter plates are preferably thin to minimize absorption of direct x-rays, and tall in the direction of the x-ray source to maximize absorption of scattered x-rays having small deviation angles. The degree of scatter rejection is improved by using plates constructed from a metal or other material with a high atomic number and by making the plates tall in the direction pointing toward the focal spot of the focus-centered detector array. In present anti-scatter elements, plates with heights of between one centimeter and four centimeters are typical.
These large anti-scatter plate heights require precise alignment of the anti-scatter elements with the spatial focal point of the detector array, and similarly precise alignment of the x-ray source at the spatial focal point. Misalignment of the anti-scatter plates can produce shadowing of the detectors by the anti-scatter plates. Shadowing, in turn, leads to reduced x-ray intensities and image artifacts which generally manifest as rings in the image reconstruction. Spatially non-uniform shadowing also leads to spectral differences in the detected x-rays and non-linear detector array characteristics. Furthermore, if the anti-scatter plates are inadequately secured, mechanical vibrations can produce temporally varying shadowing due to mechanical flexing of the tall, thin anti-scatter plates during gantry rotation which leads to a variety of image artifacts.
A conventional detector array is assembled starting with the radiation detectors, which are commonly monolithic photodiode arrays. The photodiode arrays are mounted to ceramic support substrates for rigidity, and scintillator crystals are bonded to the monolithic photodiode arrays to form detector boards. Anti-scatter elements are next mounted and aligned with the photodiodes on the detector boards. The detector boards with joined anti-scatter elements are mounted onto a mechanical base plate and manually aligned with a spatial focal spot corresponding to a convergence point of the x-ray beam. Mounting brackets for mounting the radiation detector onto the computed tomography imaging scanner are also connected to the base plate. Finally, the radiation detector is mounted onto the computed tomography scanner.
A common problem in such detector arrays is cumulative alignment stack-up errors. Accumulation of errors in alignment of the photodiode arrays, the scintillators, and the anti-scatter elements, followed by further alignment errors introduced in mounting the detector boards onto the mechanical base plate, can lead to substantial cumulative misalignment of the anti-scatter plates relative to the x-ray beam. Usually, shims, spacers, or other mechanical adjustments are provided for precisely adjusting the alignment of the anti-scatter plates of the constructed and mounted radiation detector to correct the misalignment. These mechanical adjustments are time-consuming, and the alignment accuracy of the final array is dependent upon the skill of the individual performing the anti-scatter plate adjustments.
The present invention contemplates an improved apparatus and method that overcomes the aforementioned limitations and others.
According to one aspect of the invention, a two-dimensional radiation detector is disclosed for a radiographic scanner. A support structure is provided. An alignment board secures to the support structure and includes alignment openings arranged to define a spatial focus relative to the alignment board. An anti-scatter module is disposed on the support element and includes one or more protrusions which mate with the alignment openings of the alignment board to align the anti-scatter module with the spatial focus. A detector board is provided, including a substrate and an array of radiation-sensitive elements arranged on the substrate for detecting radiation produced by the radiographic scanner. The detector board further includes alignment structures that align the detector board with the anti-scatter module.
According to another aspect of the invention, a method is provided for manufacturing a radiation detector for a computed tomography scanner. Alignment openings are defined in an alignment board. An anti-scatter element is aligned with the alignment board by mating one or more protrusions of the anti-scatter element with a selected one or more of the alignment openings of the alignment board. A detector board is aligned and mounted with the anti-scatter element. The detector board includes a substrate and an array of radiation-sensitive elements arranged thereon.
According to yet another aspect of the invention, a radiographic scanner is disclosed. A radiation source is mounted to a support frame. The radiation source emits a diverging radiation beam from a focal region. First and second generally symmetrical, substantially planar alignment boards are arranged parallel to one another and secured to the support frame. Each alignment board includes an array of alignment openings formed therein. A plurality of anti-scatter plates are arranged between the alignment boards and aligned with respect to the radiation focal region by couplings to alignment openings of both the first and the second alignment boards. A plurality of detector boards align with the anti-scatter plates.
One advantage of the present invention resides in a substantial reduction in stack-up errors in the alignment of the anti-scatter elements.
Another advantage of the present invention resides in improved accuracy in alignment of anti-scatter plates or elements.
Another advantage of the present invention resides in an improved method for manufacturing highly precise and accurate alignment plates for radiation detectors which is readily scaled to higher densities of alignment openings of various shapes and sizes.
Yet another advantage of the present invention resides in a simplified process for assembling a detector array for computed tomography imaging.
Numerous additional advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading the following detailed description of the preferred embodiment.